So there actually were two images, July and August - I didn't know that before. The frost patches differ a bit, but it would be possible to combine the two images for a 'super-resolution' or noise-reduced view.

The view here gives the impression of frosty patches atop ice-free ground, but that's an artifact of contrast enhancement. In fact, the entire scene is covered with frozen carbon dioxide.

What makes him say that? Why would there be such well-defined brighter patches if the ground was completely covered? The brighter patches don't look like south-facing slopes illuminated by the low Sun - there aren't the corresponding darker areas that you'd expect for the north facing slopes. Besides, I read that the Sun was only at one degree elevation for these images. As we know, the direct, directional illumination from the Sun would be severely attenuated by atmospheric dust when the Sun is that low. Most of the illumination would be diffuse illumination from the sky, which shouldn't produce much contrast on slopes.

Could the CO2 be continuous, but dust is covering much of the CO2, with the bright areas free of dust?

The writer also states:

QUOTE

We'll probably never know how just how much CO2 snow accumulated atop the lander by September, when the coating was likely thickest

That surprized me a bit, since spring equinox was October 26th, and with Phoenix only a few degrees north of the arctic circle, it should've been getting quite a lot of sunlight by September. But still I could believe that the July and August images show essentially the maximum amount of CO2 coverage. So maybe that's what he bases his claim on that the coverage is complete in the images?

....Most of the illumination would be diffuse illumination from the sky, which shouldn't produce much contrast on slopes.

Could the CO2 be continuous, but dust is covering much of the CO2, with the bright areas free of dust?...

It looks to me like the distinctive surface polygons are present in the dark areas but not visible in the bright areas. If the dark areas were a layer of CO2 covered by a layer of dust, they should be as smooth and featureless as the bright areas.

My question is, are the bright areas flat patches of frost or thicker drifts. If the light is entirely diffused, it may be impossible to tell anything by looking for shadows around the lander.

The coldest temperatures occurred/ended at the very end of July, based on past years' TES data. Sublimation will not exceed deposition at the minimum, but rather some time later. The temperatures come up quickly (probably with a feedback involving loss of CO2 ice), so August or maybe September is reasonable. I was surprised too, so I checked. But I imagine no substantive difference from the last image to any other image until the ice starts substantially disappearing.

My impression is the images show icy areas and non-icy areas. But, that's an impression based on an imaged stretched in ways I don't know. It could as easily be thick ice with ice or frost thin enough that the underlying soil influences the albedo. It is, in any case, an albedo contrast. I say that, not because the Sun is so low, but because you can do photoclinometry on the image. The darker areas show their topography quite clearly--I can see features Phoenix saw more clearly than in the orbiter images during the mission (although not as well as the latest pre-dark images). In my opinion, you can even see shading/brightening at the margins of the bright patches. I suggest they are likely just thicker accumulations (ie, drifts). Further, it seems that troughs appear icier than ridges.

When the Sun is very low or below the horizon, diffuse light dominates completely. But twilight skies are not even close to perfectly diffuse, and most of the light comes from 10-20 deg elevation at the solar azimuth +- 20 degrees or so (depends on exact conditions). Some USGS people have done shape from shading on orbiter images of twilit areas, IIRC.

For your perusal, here is an high resolution picture showing the Phoenix lander during winter, made from the sum-up of MRO's images ESP_014103_2485_RED.NOMAP.jp2 and ESP_014393_2485_RED.NOMAP.jp2 .It seems that the lander has lost its "symmetrical" look... (a damaged/collapsed solar panel ?) and that this effect is real and not a processing artifact.

Super resolution isn't really going to work with two push broom images. You're likely introducing more artefacts than improvements. It looks less sharp and phoenix less obvious than the orig two images I'm afraid. So drawing any conclusion from it is unwise

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